In 1986, oxides of La--Ba--Cu--O were found to exhibit superconductivity at a critical temperature (T.sub.c) as high as 30K. Since then, materials of oxides have drawn public attention as candidates for superconducting material. In 1987, it was confirmed that a series of superconductive oxides of Y--Ba--Cu--O had a critical temperature of about 90K, which is higher than the temperature of liquid nitrogen (77K). In 1988, Bi--Sr--Ca--Cu--O and Tl--Ba--Ca--Cu--O superconductors were discovered with critical temperatures exceeding 100K. Thereafter, many researchers have been able to develop superconductive materials having critical temperatures of at least 120K, such as Tl.sub.2 Ba.sub.2 Ca.sub.2 Cu.sub.3 O.sub.10 (Tl-2223 phase), which has a T.sub.c of about 120K. However, methods of preparing such materials are very delicate that it is always very difficult to prepare a material which exhibits superconductivity at temperatures exceeding 120K with satisfactory reproducibility.
In "Zero-resistance temperature of Tl-based "2223" superconductor increased to 127K," by Kaneko, T., et al., Physica C, 178 (1991) 377-382, it was demonstrated that a single-phase "2223" superconductor, i.e., Tl.sub.1.7 Ba.sub.2 Ca.sub.2.3 Cu.sub.3 O.sub.z exhibits the zero-resistance temperature of 127K and the diamagnetic temperature of 130K, when the sample was encapsulated in an evacuated quartz tube (.about.10.sup.-4 Torr) and postannealed at 750.degree. C. for 250 hours.
In "Superconductivity at 124K in (Tl, Pb)Sr.sub.2 Ca.sub.2 Cu.sub.3 O.sub.9," by Liu, R. S., et al., Physica C, 198 (1992) 318-322, it was reported that an almost pure phase (Tl.sub.0.5 Pb.sub.0.5)Sr.sub.2 Ca.sub.2 Cu.sub.3 O.sub.9 (Tl-1223 phase) can be prepared which exhibits a critical temperature of 124K by encapsulating in gold foil and sintered at 950.degree. C. for three hours in oxygen, then sealed in an evacuated quartz tube and finally annealed at 700.degree..about.750.degree. C. in an evacuated quartz tube (oxygen partial pressure .about.10.sup.-4 Torr) for 5.about.10 days.
In "An efficient and reproducibility approach for attaining superconductivity at 128K in Tl.sub.2 Ba.sub.2 Ca.sub.2 Cu.sub.3 O.sub.10-.delta.," by Liu, R. S., et al., Physica C, 182 (1991) 119-122, it was reported a method for the preparation of almost single phase Tl-2223, which is characterized by high temperature superconductivity with T.sub.c,midpoint =128K and T.sub.c,zero =126K. The procedure involves synthesizing a material with a nominal stoichiometry Tl.sub.1.6 Ba.sub.2 Ca.sub.2.4 Cu.sub.3 O.sub.10-.delta., which was sintered at 910.degree. C. for three hours, annealed at 750.degree. C. in an evacuated quartz tube for 10 days, and finally annealed at 600.degree. C. in a 0.2% oxygen/nitrogen environment for two hours.
Eur. Pat. App. No. 0 516 148 A2, which corresponds to Japanese Pat. App. No. JP 127939/91, filed May 30, 1991, discloses an oxide superconductive material having the following formula: EQU Tl.sub.X Ba.sub.2 Ca.sub.y Cu.sub.3 O.sub.z
wherein x, y and z are in relations satisfying 1.5.ltoreq.x.ltoreq.2.0, 2.0.ltoreq.y.ltoreq.2.5, x+Y=4.0, 9.0.ltoreq.z.ltoreq.11.0. The process of preparing the superconductive material include the steps of placing a mixed powder in an oxygen jet to obtain a sintered body, and annealing the sintered body in a closed atmosphere at 700.degree. to 800.degree. C. for up to 500 hours.
Eur Pat. App. No. 0 441 986 A1, which corresponds collectively to Japanese Pat. App. Nos. JP 230876/89, filed May 9, 1989, JP 252830/89, filed Sep. 27, 1989, JP 282700/89, filed Oct. 30, 1989, and JP 81834/90, filed Mar. 29, 1990) discloses an oxide superconductive material characterized by having a crystalline phase comprising oxides of the following formula: EQU Tl.sub.x Pb.sub.y Bi.sub.z Sr.sub.2 Ca.sub.y Cu.sub.3 O.sub.r
wherein x, y, and z are numbers satisfying the following condition:
x+y+z=1 PA1 0.3.ltoreq.x.ltoreq.0.95 PA1 0.ltoreq.y.ltoreq.0.5 PA1 0.05.ltoreq.z.ltoreq.0.4
and r is a number of approximately 9. The process of preparing the superconductive material include the steps of calcining a mixed powder at 850.degree. C. for 10 hours in oxygen atmosphere, pressing the mixed powder into a mold, sintering the mold at 900.degree.-950.degree. C. for 20 hours, and annealing the sintered product at three cascadingly lower temperatures for an aggregate of 60 hours. The superconductive material of oxides exhibit a critical temperature between 90 and 120K.
In the preparation of thallium-bases superconductive materials, it is well-known that the thallium atoms often escape from the powder mixture at elevated temperatures. In order to prevent a possible decomposition resulting from excess loss of thallium, it is always desirable to avoid prolonged heat treatment of the superconductive composition at elevated temperatures. Conventionally, during the annealing step, the superconductive material is sealed inside an evacuated quartz tube (.about.10.sup.-4 Torr) and annealed at a relatively low temperature of 750.degree. C. for an extended period of time, at least 200.about.250 hours. Such a long annealing time, however, will result in a weight loss of the superconductive material. It has been observed that the walls of the quartz tube often became foggy during or after the annealing step. This is due to the fact that thallium and/or oxygen can escape from the reactants in the form of thallium or thallium oxide vapor, which would react with the quartz tube or diffuse therethrough. Furthermore, even with such a prolonged annealing process, prior techniques are only able to increase the critical temperatures of Tl-1223 and Tl-2223 to 120K and 125K, respectively. It is therefore, desirable to develop a process which would allow thallium based superconductive materials, more particularly, Tl-1223 and Tl-2223, to be prepared without having to subject the same to such a prolonged annealing time.